Multivariate factor analysis of sulfur oxidation and rhodanese activity in soils

The role of rhodanese as an intermediate catalyst in the oxidation of elemental S (S°) is not well understood. This study investigated the effect of 26 soil properties and steam sterilization in relation to S° oxidation and rhodanese activity in 33 soils (27 Oregon soils and six Chinese soils). S° oxidation potential was determined by incubating (7 d at 23 °C) soil amended with 500 mg S° kg^-1 soil and measuring the SO₄ released. Both total S° oxidation (TSO) and rhodanese activity varied widely among the 33 soils, ranging from 0 to 143 mg SO₄-S kg^-1 soil 7 d^-1 and 22 to 2109 nmoles SCN^- g^-1 soil h^-1 respectively. S° oxidation but not rhodanese activity had a significant positive correlation with soil pH. In sterile soils, chemical S° oxidation (CSO) averaged 3% of the total S° oxidation and apparent rhodanese activity averaged 11% of the total rhodanese activity. S° oxidation was not significantly correlated with rhodanese activity. However, development of stepwise regression models predicting S° oxidation revealed that rhodanese activity was an important explanatory variable in predicting biological S° oxidation (TSO minus CSO). Also, microbial biomass C was found to be an important parameter in models for both S° oxidation and rhodanese activity. Investigations of the effect of acidification during S° oxidation showed that biological S° oxidation was negatively correlated with S° oxidation-induced-pH-change for soils with pH > 6 but no such significant relationship was found on soils with pH> 6. This suggested that extreme acidity may inhibit S° oxidation but not rhodanese activity.